The structure, physical properties, purity, and chemical inertness of ceramic materials such as pyrolytic boron nitride (pBN) make attractive container materials for elemental purification, compounding, and growth of semi-conductor crystals. Depending on the application and desired crucible capacity, crucibles may have a straight wall cylindrical configuration as disclosed in U.S. Pat. No. 5,158,750; a straight but tapered wall configuration as disclosed in U.S. Pat. No. 5,759,646; stepped or indented portion in its straight wall as disclosed in U.S. Pat. No. 4,946,542; or a negative draft crucible for MBE effusion cells as disclosed in U.S. Pat. No. 5,932,294 for a one-piece crucible with a narrow neck between a base section and a conical top section.
PBN crucibles are made using processes known in the art, i.e., by first preparing a mandrel having the desired shape of the crucible; depositing boron nitride upon the mandrel until the desired thickness of boron nitride is obtained; and lastly, removing the boron nitride crucible from the mandrel. Graphite is typically used as the material for the mandrel. For crucibles having a straight or tapered wall (larger top than bottom) and as graphite shrinks at a higher rate than pBN due to its thermal contraction coefficient, the graphite mandrel can slip off with relative ease to allow the removal of the pBN crucible from the mandrel. However, for crucibles with indented or curved portions, e.g., a neck in the crucible wall, it is necessary to design special mandrels with that fracture into pieces as disclosed in U.S. Pat. No. 5,827,371 to remove the top portion of the mandrel. With respect to the bottom part, an additional process step of heating the crucible to 300-750° C. for 40 hours to oxidize the bottom portion of the mandrel, thus destroying the graphite mandrel bottom part and yielding a unibody, monolithic pBN crucible.
The invention relates to a ceramic crucible comprising a plurality of pieces, forming a unibody, and a method for making a unibody crucible from a plurality of pieces without the need for a heating step to destroy and remove the complex graphite mandrel from the crucible.